A diffusive sampler is in concept a simple device for determining the presence of a chemical substance--the "analyte"--in a fluid by exposing the device to the fluid so that analyte carried by the fluid can be sorbed by an sorbent in the device, thereby being bound in the sampler. These devices are also called "air samplers" or "gas samplers" because of their use in sampling airborne, gaseous chemicals.
Diffusive samplers are commonly used in the workplace for monitoring workers' exposures to toxic gases and vapors. They can be inexpensive to manufacture, small and convenient to wear, and are often simple to analyze.
The use of diffusive sampling to determine the concentration of toxic chemicals in the workplace goes back many decades. For example, in Methods of Air Analysis published by Charles Griffen and Co. in 1912, J. S. Haldane reported that "The presence of sulphuretted hydrogen [in current chemical terminology, hydrogen sulfide] can easily be recognized chemically by the blackening of a strip of paper which has been dipped into acetate of lead solution. Any air in which this blackening occurs within a minute should be regarded as very dangerous." He also reported that: (1) nitrous fumes could be sensed using a strip of paper moistened with a solution of starch and potassium iodide, which solution turns blue in the presence of the nitrous fumes, and (2) phosphine blackens paper dipped in silver nitrate.
Around 1952, the Central Intelligence Agency used a type of diffusive sampler it called a "sneaky" which was a chemically treated handerchief. After exposure to the air for a minute, the handkerchief would pick up factory fumes that could later be analyzed and identified. However well-suited sneakies were for determining the presence of unknown substances, their ability to yield information about concentrations of those substances was very limited.
In environmental monitoring, diffusive samplers are also well known. For example, it was once common to measure environmental sulfur dioxide by coating a tube or plate with lead peroxide and exposing the plate to the environment for several weeks to several months. If sulphur dioxide were present in that environment, it would react with the sulphur to form lead sulfate, the presence and concentration of which could be detected and quantified by analysis. However, after more accurate analytical techniques were developed, it was clear that there was little correlation between these newer techniques and the diffusive sampler for sulphur dioxide.
Although there were a number of attempts to increase the accuracy of these diffusive samplers for sulphur dioxide, none was successful because the accuracy of the sampler depended on a complete understanding of the interaction of the analyte and the surface of the sampler, a fact not fully appreciated at the time. Diffusion into a material depends intimately on the nature of the surface of that material. Furthermore, reproducing the nature of a lead oxide surface requires considerable technical capability.
Although diffusive samplers have a number of advantages, the inability to accurately quantify the concentration of the analyte had limited their use and effectiveness. Clearly important were the variations in air flow where the sampler was used. Mass transfer through the laminar flow of air adjacent to the sampler surface depends on the thickness of the laminar layer, which in turn depends on the turbulent flow of air beyond the laminar layer.
Another source of errors is exposure to the air after the sampling period. Because analytes may also desorb when the sampler is exposed to the air after the sampling period, the diffusive sampler should be packaged or sealed in a vapor-proof package as soon as the sampling period ends.
There have been some attempts to solve these problems. E. D. Palmes and G. D. Gunnison in "Personal Monitoring Device for Gaseous Contaminants," published in the American Industrial Hygiene Association Journal, Vol. 32, pages 78-81 (1973), advanced the art in their diffusive sampler for nitrogen dioxide. They found that sampling accuracy can be improved by incorporating an internal diffusive mass transfer resistance (which can be maintained constant) which is greater than, and therefore suppresses the effects of, the variable external diffusive mass transfer resistance. For this purpose, their device (often called a "Palmes'tube") contained a quiescent air gap in front of the sampling substrate to limit the uptake rate to: EQU W=DACt/L,
where "W" is the uptake in micrograms, "D" is the diffusion coefficient for the analyte in air in square centimeters/second, "A" is the exposed sampling surface, "C" is the average airborne concentration of the analyte in micrograms per milliliter, "t" is the sampling period in seconds, and "L" is the length of the air gap in centimeters. After exposure and analysis, all factors except for the concentration "C" are known, and therefore "C" can be determined from the known factors using this equation. It is clear that the improvements of Palmes and Gunnison included both the use of the gap that made uptake of the analyte proportional to "t", the duration of exposure, and the use of a layer of known material--air, in particular--wherein the diffusion coefficients of many important compounds are already well known or easily measured.
Accuracy can be and also has been improved by impregnating the sampler with two sorbents having different sorption capacities, with the sorbent having the lower sorption capacity placed on the exterior surface. This arrangement is more effective when used in diffusive samplers intended to sample a very wide variety of contaminants having different molecular weights. Higher molecular weight contaminants are retained on the exposed layer, and the lower molecular weight contaminants penetrate to the interior layer. In the absence of this arrangement, one would have to select a sorbent that either (because of its low capacity) did not retain low molecular weight compounds, or (because of its high capacity) would not release the high molecular weight compounds for further analyses.
Locker, in U.S. Pat. No. 4,327,575, describes the use of two sorbent layers wherein the outer layer is used to remove undesirable components of the atmosphere before the analyte of interest reaches the inner layer.
Related diffusive devices are designed to emit analyte at a constant rate. They rely on Fick's first law of diffusion, known since the nineteenth century, which states that if the concentration difference across a membrane is held constant, then the steady state diffusion of analyte across the membrane will also be constrant. Devices incorporating this basic principle include:
1. Permeation tubes, in which (usually) a liquid aliquot of the analyte is held in a polytetrafluoroethylene tube, through which it diffuses very slowly (See page 130 of The Industrial Environment--Its Evaluation and Control, U.S. Department of Health, education and Welfare. 1993. Stock Number 017-001-0396-4). To maintain a constant emission rate, the tube is generally placed in an oven where the temperature can be carefully controlled. PA1 2. Diffusion tubes, in which (usually) a liquid aliquot is placed in a glass bulb connected to a long necked stem. The loss of analyte is controlled by the vapor pressure of the analyte, the diffusion coefficient of its vapor in the ambient atmosphere, and the geometry of the long necked stem, An excellent description of this procedure is given by Altshuller and Cohen in Application of Diffusion Cells to the Production of Known Concentrations of Hydrocarbons, Analytical Chemistry 32:802 (1960). PA1 3. Saturated beds of adsorbent (see U.S Pat. No. 4,445,364, Stieff et al.) that constantly lose analyte, thereby introducing a steady flow of analyte into an air stream.
None of these devices was designed for, or would be expected to be useful for, the sampling of analytes from the atmosphere.
This review of these diffusion-based devices shows that there remains a need for an improved diffusive sampler that can be used to determine more accurately the presence and the concentration of the analyte.